WEAK LONG-DISTANCE SUPEREXCHANGE INTERACTION AND ITS TEMPERATURE-VARIATIONS IN COPPER(II) COMPOUNDS STUDIED BY SINGLE-CRYSTAL EPR

In this paper we review experimental EPR data and theoretical models of the weak superexchange interaction (J < 0.1 cm(-1)) mainly in copper(II) compounds single crystals. The different EPR techniques are described which allow one to determine the exchange coupling such weak as J = 0.0001 cm(-1). Such a weak interaction is detectable by EPR on distances up to 25 Angstrom and the presented data allowed us to establish empirical limits for a weak exchange coupling transmission for J < 0.3 cm(-1) as \J(iim)\ = 5.9exp(-0.335R) where R (Angstrom) is the intermolecular distance. The weak superexchange coupling was found to be temperature dependent and this dependence is very specific to a crystal or molecular structure. In most cases temperature dependence of superexchange is governed by thermal crystal lattice contraction. A phenomenological model of competing potential and kinetic exchange is presented and used for description of copper(II) compounds with opposite temperature behaviour of superexchange coupling. Possible mechanisms driving temperature variations of superexchange are discussed.